To prepare a surface for electroless plating, such as electroless copper plating, one first cleans the surface very thoroughly. The surface is then given a treatment that activates it for subsequent electroless plating, by forming catalytic deposition sites on the surface. Then the copper will deposit electrolessly onto the surface simply by immersion in an appropriate electroless copper plating solution. The activation treatment referred to involves soaking the surface a few minutes in a costly activator solution. The activator solution is normally a colloid of palladium chloride and stannous chloride, that is highly sensitive to oxidizing contaminants and to reduction in its chloride ion concentration. On the other hand such contaminants and water can easily be dragged into the activator bath from prior process steps. Accordingly, one customarily protects the activator bath by first immersing a part to be plated in a less costly, i.e. predip, bath that is specially formulated with hydrochloric acid and sodium chloride. When contaminants detrimental to the activator solution build up in the predip bath, the predip bath is dumped. Thus no significant contaminant concentration is dragged into the activator solution to quickly poison it.
As mentioned, oxidizing agents can poison the activator solution. Peroxides, copper ions in the plus two oxidation state (Cu.sup.+2), and chromium ions in the plus six oxidation state (Cr.sup.+6) serve as oxidizing agents in the activator solution.
One may use electroless copper plating to form conductive passages extending between opposed copper plated faces of a resin based electronic circuit board. Such plated passages are sometimes referred to as plated-thru-holes. Part of the cleaning process in making a plated-thru-hole in such a circuit board involves etching the copper faces of the circuit board. Unless thoroughly rinsed, copper ions in the plus two oxidation state can adhere to the face of the circuit board, and be dragged into the activator bath. As mentioned, this can degrade it. Accordingly, immersion in a predip bath is customarily used prior to immersion of the circuit board in the activator bath.
When plating on plastics, the plastic is frequently initially etched in a strong oxidizing solution, as for example chromic acid. Even though one may attempt to rinse off all of the oxidizing agent, i.e. the chromic acid, from the surface, some of it may still adhere to the surface. If the chromium ions are oxidized to the plus six oxidation state, and enough are dragged into the activator bath, they might poison it. Hence, use of a predip bath is important when plating on plastics.
We have found how to render the oxidizing agents in a predip bath less harmful. Thus, we have found a process by which the lifetime of the predip bath can be extended before it must be dumped and replaced. This reduces the overall volume of predip bath that must be dumped, reduces the man hours used and production time lost when rebuilding fresh predip baths, and reduces the cost of constituents used in the predip bath.